This invention relates to methods for making depressed index optical fibers and is directed more specifically to techniques for preparing preforms prior to fiber draw.
Depressed clad optical fibers were developed in the early 1980""s as an alternative to fibers with doped cores and less heavily doped, or undoped cladding. See, e.g., U.S. Pat. No. 4,439,007. Depressed cladding allows the use of fiber cores with relatively low doping, or no doping at all. These cores produce low optical loss.
Applications were developed for both single mode and multimode depressed clad fibers, and a variety of processes for the manufacture of depressed clad fibers were also developed. See e.g. U.S. Pat. No. 4,691,990, the disclosure of which is incorporated herein by reference.
Recently, there has been a renewed interest in depressed clad fibers for lightwave systems in which control of non-linear effects is important. For example, in four-wave mixing of optical frequencies in the 1.5-1.6 mm wavelength region where DWDM networks operate, a low slope, low dispersion fiber is required. A fiber structure that meets this requirement is one with multiple claddings including one or more of down-doped silica.
One technique for making depressed clad fibers is to dope the cladding of a silica core fiber with fluorine or boron, which produces cladding with a refractive index less than the silica core. For example, fibers with negative refractive index variations, xcex94n, in the range 0.05-0.7% can be obtained using fluorine doping.
More recently, fibers with down doped core regions have been proposed which have a core shell doped with fluorine and a center region doped with a conventional dopant such as germania. This produces a modified xe2x80x9cWxe2x80x9d index profile and is found to be desirable for dispersion control. Manufacture of these fibers typically involves a standard VAD process, but the process is complicated by the step of selectively doping the shell region with fluorine. Fluorine diffuses readily into the porous structure and it is difficult to prevent fluorine migration into the germania doped core region.
Fibers with depressed index cores or cladding can be produced using any of the conventional optical fiber production techniques, which include rod in tube processes, MCVD and PCVD (inside tube deposition processes), and VAD or OVD (outside tube deposition processes). For single mode depressed clad fibers, the rod-in-tube approach may be preferred due to the large amount of cladding material required. Preforms for these fibers require a high quality, low loss cladding tube. For fluorine doped cladding, a desirable option for preparing the fluorine doped cladding tube is by a sol-gel process. This process is described in detail in U.S. Pat. No. 5,240,488, which is incorporated herein by reference for those process details, In the sol-gel process a porous silica tube is produced, and then heated to consolidate it into a solid (vitreous) cladding tube. The rod is then inserted and the tube collapsed around the rod. Depressed clad optical fiber preforms can also be made using vapor axial deposition (VAD), or outside vapor deposition (OVD). Each of these processes produces an intermediate product which is a shaped porous particulate material that is then consolidated into a preform. For the purpose of this invention, the common ingredient in all of these techniques is that, at one stage in the preform fabrication process, the preform is a highly porous silica body. Porosity of these bodies is typically in the range of 50-90%, measured as volume of the voids to total volume of the body.
In the manufacture of fluorine doped preforms, it is convenient to dope the porous silica body with fluorine by xe2x80x9csoakingxe2x80x9d the cladding tube in a fluorine containing gas atmosphere with the cladding tube still in the porous state, i.e. prior to consolidation. The porosity of the cladding tube at this stage in the process allows the fluorine gas to easily permeate through the entire thickness of the wall of a cladding tube, and through the thickness of a core rod. The conventional practice is to diffuse fluorine into the silica body using an equilibrium doping process. In this process, the silica body is heated to a temperature of rapid diffusion, in the presence of a low partial pressure of fluorine, i.e. a partial pressure sufficient to supply a continuous flux of fluorine to maintain the equilibrium. For the stated An values, equilibrium requires low fluorine partial pressures (10xe2x88x923-10xe2x88x924 atmospheres). Using such low values limit the rate at which F can be incorporated into the body, thereby requiring excessively long process times, e.g. greater than 20 hours for even modest sized preforms. In some cases the high temperature soak using the equilibrium doping technique causes partial, premature consolidation of the porous cladding tube so that a gas equilibrium state is not reached before the pores close in the exterior portion of the porous cladding tube close. This prevents complete equilibrium of fluorine through the porous body, and the required low partial pressures of the equilibrium doping process, slows the incorporation to the point where unacceptably long process periods are required. For thick clad tubes, the processing times may be entirely prohibitive.
We have discovered a fluorine doping process for optical fiber preforms that accelerates the overall doping process by at least an order of magnitude. In this new process the surface regions of the individual particles in the porous body are deliberately overdoped, and the final desired doping level is achieved by solid/solid diffusion to the desired doping level. The process is characterized in part by effecting the fluorine diffusion step into the particles in an atmosphere free of fluorine. We refer to this process as incremental doping to distinguish it from the conventional equilibrium doping process.